Watch Talk: What’s the Beat Error of a Watch?

 In Watch Talk

The beat error is a hot item on many watch-related forums.

If you look it up, you might find something like: “beat error is an asymmetrical oscillation of the balance wheel.”

In this article, I’ll explain what it is and why it’s important.

What is it?

As you know, the balance wheel swings clockwise and counterclockwise. Each one of those swings, in either direction, is called a “beat.” If the watch runs perfectly, the clockwise swing will be the same as the counterclockwise swing, and both swings will take the same amount of time.

The beat error is the difference between the clockwise swing and the counterclockwise swing of the balance. In perfect conditions, the beat error is 0.

However, perfect conditions don’t exist in the real world.

In reality, the clockwise swing, for example, will take a bit longer than the counterclockwise swing. The watch is slightly “out of beat.”

In the example above, the beat error could be 1.2 ms (it’s measured in milliseconds). That means that the clockwise swing takes 1.2 ms longer than the counterclockwise swing.

Do you have a watch that doesn’t start ticking on its own but needs some “help”? That might be due to a high beat error.

What causes it?

The balance wheel swings to the left and the right. Halfway, the impulse pin of the balance enters and exits the fork of the lever.

The balance wheel is yellow. The lever and escape wheel are blue. The pallet stones and impulse pin are red

The lever is placed between two banking pins.

Beat error

You can see the lever (grey-blue) in between the banking pins. The pallet stones are red, and the escapement wheel is gold. You can see the fork at the end of the lever

The zero position of the balance is the position in rest without any power in the mainspring. The location of the impulse pin is most important because that’s the middle point between a clockwise swing and a counterclockwise swing.

The dead center of the banking pins is the ideal resting place for the impulse pin in the zero position. The beat error would be 0.

However, it’s nearly impossible to achieve this so the impulse pin will likely be a little to the left or right of this point. That causes a beat error because one direction of the swing is slightly longer than the other one.

Remember that we’re talking milliseconds here, so the differences are minimal.

How do you correct it?

Mobile stud carrier

If you’re lucky, your watch has a mobile stud carrier. With it, you can quickly reposition the complete balance and therefore the position of the impulse pin in relation to the banking pins.

Place your watch on a timing machine so you can manipulate the mobile stud carrier and monitor the beat error at the same time.

Beat error + mobile stud carrier

Number 2 is the mobile stud carrier. Number 1 is the regulator with the index pins (to adjust the rate)

Fixed stud carrier

If the movement doesn’t have a mobile stud carrier, you’ll need to move the collet of the hairspring on the balance staff. That will change the position of the impulse pin in the zero position as well.

Always remove the balance and place it on a balance tack if you want to move the collet. You can use a tiny screwdriver, or you can use a special hairspring collet tool like the Bergeon 30017.

As you can imagine, this is a very fiddly job so you should only consider this when it’s necessary. Otherwise, it’s not worth the risk of damaging the hairspring, especially if you know that spare parts are impossible to find.

What’s a good beat error?

To be clear, the beat error on its own means very little. You should always consider the amplitude and the rate as well.

A watch with a high beat error can still keep accurate time if it has a strong amplitude and a consistent daily rate. However, it’ll be more vulnerable to problems with timekeeping when the amplitude is low or during sudden changes in the position of the watch.

If your movement has a mobile stud carrier, you can easily adjust the beat error to 0.2 ms or 0.3 ms. If it has a fixed stud carrier, a beat error of 0.8 ms and below is fine.

Have you ever corrected the beat error? If you have any more tips, please leave them in the comments below.

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Showing 10 comments
  • Avatar
    Joshua
    Reply

    This was very clear, concise, and helpful, thanks!

  • Avatar
    Russ Kawai
    Reply

    I agree with Melvin, very helpful. One question, a close friend of mine received a watch but it was running slow (about 5-10 minutes per day). He put it on a timegrapher and found that the beat error was high. So he tried to fix the beat error first before adjusting the rate. Unfortunately, while adjusting the Mobile Stud Carrier (MSC), the watch stopped running. He wanted to know if its possible to adjust the MSC too much such that the watch stops completely.

    • Melvin Hollenberg
      Melvin Hollenberg
      Reply

      Hi, yes that’s possible. A beat error is measured in m/s so it only needs tiny adjustments. It’s a good idea to look up pictures of the movement online to see the general position of the stud. From there, only small corrections are needed. By the way, a rate of -5 to -10 minutes a day is huge.

  • Avatar
    Jim Gill
    Reply

    If a brand new watch has a beat error of 1.1, should it be sent back?

    • Melvin Hollenberg
      Melvin Hollenberg
      Reply

      A beat error of 1.1 is too much. I don’t know if the best action would be to send it back though. An independent watchmaker with a timing machine could easily fix it.

  • Avatar
    Matt
    Reply

    Very good article. I also have a question. Does the beat error increase as the power of the mainspring decreases? I have a watch that has used most of it’s reserve sitting on the bench. The amplitude and speed are good, 240 and +7. The beat error has gone from 0.2 to 2.5. Thanks

    • Melvin Hollenberg
      Melvin Hollenberg
      Reply

      No, it shouldn’t increase because the beat error refers to the position of the impulse pin in rest and the difference in swings. That has nothing to do with the power reserve, especially since the rate and the amplitude are still good. Something else must have caused the beat error to increase.

  • Avatar
    Matt
    Reply

    Thank you for your input. I serviced this movement. All the gears in the drivetrain were clean and straight. I’m pretty sure I oiled everything correctly. I oiled just the flat spots on the pallet fork after I cleaned it. I cleaned the balance assy and oiled the jewel in the balance cock. It runs so good under full power that I’m left scratching my head about the change in beat error. Any suggestion on where to look would be greatly appreciated.

    • Melvin Hollenberg
      Melvin Hollenberg
      Reply

      Sorry, I’m at a loss as well. I’ve never seen a sudden change in beat error like that. I’d remove the balance and clean the balance pivots with jewelers rouge or something. Then, clean it (the cap jewels as well) and lubricate again. Did you check if the banking pins are ok? Are the regulator pins ok? Is the hairspring flat and not touching anything? Are all the windings of the hairspring concentric without touching each other?

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Learn what is the amplitude of a watchPaul Altieri from Bob's Watches